The increasing cost and shortages of liquid fuels, especially gasoline, have turned attention to alternate operation of liquid fueled engines on gaseous fuels, such as propane, which are relatively much less expensive and more available. This is particularly true in the case of larger vehicles such as trucks, motor homes and the like which can more readily afford the extra space to carry the large pressurized tanks of gaseous fuel in liquid state. Alternate engine operation on gaseous fuels, though long known and practiced, nevertheless suffers from several longstanding problems somewhat inherent in the nature of those fuels. One of those problems is that of obtaining acceptable slow-run operation during idle and progressive off-idle conditions, especially when the throttle is suddenly opened. Current practice usually provides one system for high speed operation and another for slow-run, much as in liquid fuel carburetors, except that often the two in gaseous fuel installations are not nearly as integrated with each other as they are in liquid fuel instances.
One common slow-run system takes the gas from the main fuel line between the zero pressure governor and the main fuel inlet at the venturi and passes it through a separate line and metering valve to the engine's intake manifold or other point downstream of the throttle. The high intake manifold vacuum during slow-run conditions activates the zero pressure governor to supply gas to the intake manifold, the latter supply gradually tapering off as the main fuel system comes into operation as engine speed increases. The chief trouble with this arrangement is that it tends to produce a flat spot when the throttle is suddenly opened. The consequent sharp drop in manifold vacuum starves the slow-run system by allowing the zero pressure governor to close down until engine speed is sufficient to reactivate it by virtue of the demand communicated to it by the main fuel system and by the partial recovery of vacuum in the intake manifold. In addition, in part owing to the restriction furnished by the metering valve in the slow-run line and in part depending upon how "sensitively" set the zero pressure governor is, sudden throttle opening may initially also pull air in from the main fuel inlet and thence into the slow-run system rather than activating the zero pressure governor to supply fuel to the slow-run and the main systems, thus adding to the severity and duration of the flat spot. In short, in the foregoing system operation of the zero pressure governor is disturbed, with the foregoing adverse consequences, when progressing from idle to off-idle conditions.
Another well known slow-run system eliminates the separate line to the intake manifold and instead sets the zero pressure governor so that it remains open sufficiently to supply fuel for slow-run purposes through the main fuel line to the venturi. In effect, during slow-run conditions the zero pressure governor acts as a pressure regulator rather than as a demand regulator. This arrangement is sometimes referred to as a "leaker" system inasmuch as the zero pressure governor continuously "leaks" gas into the main fuel line during slow-run. The "leaker" system also has its own deficiencies. One of these is that it tends to impose a back pressure upon the gas through the main fuel inlet at the venturi which in turn inverts the diaphragm of the zero pressure governor and thus upsets idle and off-idle conditions. This may also be aggravated by the metering restriction interposed in the main line to adjust the slow-run engine speed. Hence, if the throttle is suddenly opened, there can be an appreciable lag before the zero pressure governor can "reconvert" itself from "pressure" to demand type operation and supply sufficient fuel to the venturi. In short, the "leaker" system also disturbs proper operation of the zero pressure governor.
Still another system brings the slow-run gas into the liquid fuel carburetor below the throttle through a line incorporating a transverse rotary valve linked to the throttle such that the valve opens to supply more gas whenever the throttle opens. This functions fairly well and progressively but is difficult to incorporate into the liquid fuel carburetor besides requiring a considerable number of rather expensive, precision parts as well as a mechanical linkage. Other approaches more completely integrate the main and slow-run systems. One employs a diaphragm governed valve which controls a small variable venturi for slow-run and separate fixed large venturi for high speed operation, and another uses a diaphragm controlled valve in conjunction with a fuel valve which together meter both air and fuel throughout the engine's speed range including slow-run. Examples of these two latter approaches are found in U.S. Pat. Nos. 2,937,848 and 2,983,592 from which it will be readily seen that both are fairly intricate and thus expensive and elaborate to manufacture.
Accordingly, the primary object of the present invention is provide a slow-run system for gaseous fuel operation of an internal combustion engine which avoids the foregoing problems and deficiencies on the one hand but which on the other is simple in structure, inexpensive to produce, efficient in operation, and readily incorporated in a gaseous fueled engine or in a conversion of a liquid fueled engine for alternate gaseous fuel operation.